The present invention pertains to methods and device for testing or measuring segregation of constituents of a mixture of particulate solids. In particular, the invention are photometric methods and devices for measuring both the quantity and spatial dispersion of different particles in a mixture of various solid particles.
In a great variety of industries and technologies mixtures of solid materials are known to segregate. Segregation is relative movement of one or more of the different solid constituents within a mixture resulting in spatial stratification and non-uniformity within a volume of the mixture. Segregation can occur in previously uniform mixtures as a result of a variety of events or forces occurring in transportation and handling and are typically induced by differences in particles size, shape, density and other mechanical and chemical properties.
In process design, the solution to a segregation problem can be attacked from two angles. The process can be modified to accommodate segregation patterns caused by the various mechanisms or changes can be made to the process to reduce the cause of segregation. In either case, understanding segregation mechanisms is critical to developing robust processes to handle segregating materials.
Materials segregate when handled for a variety of reasons. Many solids flow practitioners quickly identify the potential for fine material to sift through the matrix of coarse particles as material slides down a pile. Indeed, sifting segregation is a predominant cause of separation during handling of differently sized particles. This mechanism usually results in a radial segregation pattern where fines accumulate near the center of a pile while the coarse material is predominately at the pile's edge. However, severe sifting segregation can cause a top-to-bottom segregation pattern where the fines are beneath the coarse particles. This is especially true if inter-particle motion is induced within the material by some external means such as vibration. Typically, particle size differences greater than three to one are enough to produce significant sifting segregation problems.
As well, some particles have differences in inter-particle friction and thus form piles with different repose angles. Formation of piles within process equipment causes the less frictional particle to slide further down the pile accumulation at the pile's edge. This mechanism results in a radial segregation pattern. Materials with an angle of repose difference of more than 2 degrees can show significant repose angle segregation.
Air currents caused during filling may carry fine material to regions where the air currents decrease sufficiently to deposit the fine material. This air entrainment segregation can produce a radial pattern or a side-to-side pattern depending on the position of the inlet and the geometry of the vessel. Generally, fines accumulate near process vessel walls with this segregation.
Other events can also result in segregation of materials within. Segregation is a problem because of the resulting uncertainty and or nonuniformity of the materials when applied or used in processes. In a simple example: segregation of bulk mixtures of baking ingredients in industrial food production can result in significant loss of quality control.
Knowing the segregation mechanism as well as the flow profiles in process equipment is critical to solving segregation problems. In typical existing segregation test methods, mixture samples are taken from process streams of material, and the constituents physically measured, in order to learn the segregation parameters. However, these methods are limited in application and inaccurate. Methods of accurately measuring the magnitude and type of segregation occurring in material process systems are needed.